1. Field of the Invention
The present invention refers to frequency synthesisers, in particular to multi-band frequency synthesisers used in mobile terminals for wireless telecommunication networks.
2. Description of the Related Art
New generation mobile terminal are equipped to support data transmission on UMTS (Universal Mobile Telecommunications System) and GSM/GPRS (Global System for Mobile Communication/General Packet Radio Services) frequency bands in one body. Therefore multi-band frequency synthesiser are required which support UMTS-FDD (UMTS-Frequency Division Duplex) and UMTS-TDD (UMTS-Time Division Duplex) frequency bands as well as triple bands functionality (GSM900/DCS1800/PCS1900 MHz) for GSM/GPRS in both up- (Tx) and down-link (Rx) directions. DCS hereby stands for Digital Cellular System and PCS for Personal Communications Service.
Today""s known implementations of multi-band frequency synthesisers are based on the usage of independent frequency synthesisers, one for UMTSxe2x80x94up-link, one for UMTSxe2x80x94down-link and a third one for triple band GSM/GPRS. To achieve the necessary class 12 settling time for GPRS (below 150 xcexcs) Fractional-N synthesisers are commonly used.
The biggest advantage of Fractional-N frequency synthesisers is their fast settling time when e.g. switching from an up-link to a down-link frequency band or at an intersystem handover, which fulfils one of the preconditions for a reliable GPRS data transmission. The biggest disadvantage of all classes of Fractional-N frequency synthesisers is their bad output spectrum performance which is characterised by high phase noise and a high amount of spurious spectra. Especially when receiving low-level signals, this can be very problematic. When using Fractional-N frequency synthesisers, the requirements of GSM standard 0505 can only be achieved by post-processing the output spectra with high-quality filters, which are bulky and cost-intensive. This collides with the general requirements for a compact design of mobile terminals not leaving much space for bulky components, and for cost-effective technical solutions.
It is therefore an object of the present invention to propose a cost-effective multi-band frequency synthesiser with minimal space requirements but with a short settling time and with very low phase noise and a very low portion of spurious spectra.
This object is achieved by a frequency synthesiser arrangement and a mobile terminal as claimed in the respective independent claims.
In particular, the above object is achieved by a frequency synthesiser arrangement for generating signals with frequencies for UMTS and GSM/GPRS frequency bands, comprising a reference frequency source for providing a signal of constant reference frequency, a first frequency synthesiser sub-unit for transforming the signal of the reference frequency source into a signal with a frequency in a range of a first type of frequency band, a second frequency synthesiser sub-unit for transforming the signal of the reference frequency source into a signal with a frequency in a range of a second type of frequency band, whereby the second frequency synthesiser sub-unit further transforms the signal of the reference frequency source into a signal with an intermediate frequency, and a third frequency synthesiser sub-unit transforms the signal of the reference frequency source into an auxiliary signal with a fixed frequency, i.e. small frequency range, which is used together with the signal of intermediate frequency for generating signals with frequencies in a range of a third and of a fourth type of frequency band.
The first, second, third and fourth type of frequency bands are different from each other and correspond to the frequency bands necessary for different wireless communication standards. In particular, the first type of frequency band supports the UMTS TDD1, FDD-Tx and TDD2 standards, the second type of frequency band supports the UMTS FDD-Rx standard, the third type of frequency band supports the GSM/GPRS 1800/1900 MHz standards DCS and PCS, and the fourth type of frequency band supports the GSM 900 MHz standard.
The above object is further achieved by a mobile terminal for use with UMTS and GSM/GPRS frequency bands with a frequency synthesiser arrangement according to the present invention.
The solution of the present invention advantageously provides a multi-band frequency synthesiser for communication on UMTS/GSM/GPRS frequency bands of compact design, fast switching behaviour, with no spurious spectra produced in an unwanted frequency band, and which shows low phase noise. Furthermore, by dispensing complex components in the design, a cost-effective solution is achieved.
Additional advantageous features of the present invention are claimed in the respective sub-claims.
The frequency synthesiser sub-units advantageously comprise a frequency divider for transforming the signal of the reference frequency source into a scaled-down signal which frequency value represents the reference frequency divided by an integer value, and the frequency synthesiser sub-units comprise further advantageously a phase locked loop frequency synthesiser for transforming the scaled-down signal into a signal which frequency is an integer multiple of the scaled-down signal.
A frequency mixer preferably mixes a signal from the second synthesiser sub-unit with a signal derived from the auxiliary signal for forming a signal in a range of the third type of frequency band or for forming a precursor for deriving a signal in a range of the fourth type of frequency band. Advantageously, a frequency divider divides the frequency of the auxiliary signal by an integer value before it is being applied to the frequency mixer.
In a preferred embodiment of the present invention, a frequency divider derives a signal in a range of the fourth type of frequency band from a signal supplied by the frequency mixer by dividing the frequency of the signal supplied by the frequency mixer with an integer value.
A low pass filter is preferably used to eliminate harmonics created in the circuitry for generating signals used for a signal with a frequency in a range of the third or the fourth type of frequency band, and further preferably a low pass filter is used to eliminate harmonics created by the frequency divider providing the signals for the fourth type of frequency band.
The frequencies of the first type of frequency band may advantageously take on values from one of the frequency intervals ranging from 1900 MHz to 1980 MHz and 2010 MHz to 2025 MHz, the frequencies of the second type of frequency band may further advantageously take on values from the frequency interval ranging from 2110 MHz to 2170 MHz, the frequencies of the third type of frequency bands may preferably take on values from one of the frequency bands situated within an interval ranging from 1710 MHz to 1990 MHz, and further preferably, the frequencies of the fourth type of frequency bands may take on values from one of the frequency bands situated within an interval ranging from 880 MHz to 960 MHz.
The present invention may advantageously be implemented on mobile phones particularly for wireless cellular telecommunication networks,